This invention relates generally to radiation imagers, and more particularly, to the incorporation of a light block material to reduce cross-talk caused by persistent transistor photoconductivity, nonlinear pixel response, and poor contact via integrity.
Radiation imagers are typically coupled with a scintillator, wherein radiation (such as an x-ray beam, for example) absorbed in the scintillator emits optical photons which in turn pass into a light sensitive region of the imager. The imager typically comprises a significantly flat substrate (e.g., glass) on which a two dimensional array of light-sensitive pixels is disposed. Each pixel comprises a light-sensitive imaging element (photosensor), such as a photodiode, and an associated switching element, such as a thin film transistor (TFT). Both photodiodes and TFTs preferably comprise hydrogenated amorphous silicon (doped or undoped) or alloys thereof, due to the advantageous characteristics and relative ease of fabrication associated with these materials. Hydrogenated amorphous silicon is commonly referred to as xe2x80x9camorphous siliconxe2x80x9d or xe2x80x9ca-Sixe2x80x9d, and the light sensitive pixel array discussed above is typically referred to as being xe2x80x9cactivexe2x80x9d. Also contained in the active area of the imager are metal address lines electrically connected to the pixels.
A reverse-bias voltage is applied across each photodiode. Charge generated in the photodiode as a result of the absorption of light photons from the scintillator is collected by the contacts, thereby reducing the bias across the diode. This collected charge is read when the TFT switching device in the array couples the photodiode to readout electronics via an address line.
The address lines of the active array are electrically contiguous with contact fingers extending away from the active pixel region toward the edges of the substrate, where they are in turn electrically connected to contact pads, typically through contact vias. Electrical connection to external scan line drive and data line read out circuitry is made at the contact pads.
In the active region, optimal spatial resolution and contrast in the signal generated by the array is achieved when incident optical photons from the scintillator are absorbed substantially only in the photodiodes directly in line with the region of the scintillator in which the optical photons are generated. However, optical photons from the scintillator are often scattered, passing into the TFT switching devices or the address lines. Such scattering and absorption present problems of increased cross-talk and noise in the array. Cross-talk reduces the spatial resolution of the array, and absorption of optical photons in TFT switching devices can result in spurious signals being passed to the readout electronics.
Thus, although light from the scintillator discharges the reverse bias of the diode (the desired signal), the same light also impinges on the a-silicon in the TFT, producing a photocurrent, which is driven by the high source-drain voltage. This photocurrent persists even after the light is terminated. Hence, when pixels in the area under the object are read, even after the x-ray beam or other radiation is turned off, some of the persistent photocurrent from pixels not being read is integrated by the readout amplifier. Thus, an undesirable type of long range spatial cross-talk in the image is produced. An additional impact of the photocurrent is the production of a non-linear response by the pixel, which occurs because charge is lost from the photodiode due to this leakage current.
It is therefore clear that TFT photosensitivity can degrade the performance of an a-Si radiation imager, such as an x-ray imager. Furthermore, at high, but clinically relevant, x-ray exposure levels, for example, photodiodes in pixels which do not have the x-ray signal attenuated by the object under examination develop the highest field effect transistor (FET) source-drain voltages, thus increasing the FET photocurrent.
In addition to the aforementioned problems, another concern is that the integrity of the contact vias, which are located in the contact fingers outside the active region of the array, is often compromised during fabrication and processing. The contact vias are filled with a common electrode material comprising a light-transmissive conducting oxide, typically indium tin oxide (ITO), tin oxide, indium oxide, zinc oxide, or the like. External electrical connection from the contact pads to the underlying metals of the address lines extending from the active array is facilitated through the common electrode in the contact vias.
However, the relatively thin (about 100 nm) common electrode layer must form a continuous layer over an underlying, relatively thick (1 to 2 xcexcm) light-transmissive dielectric layer in the contact vias. Because the ITO is so thin and because it may be porous due to its polycrystalline nature, contact to the underlying address line material in the contact fingers may be degraded during processing, particularly by chemical attack of the underlying metal and ITO-metal interface.
It is therefore clear that an imager array in which the TFTs are shielded from incident optical photons is desirable. Furthermore, improvement in the electrical yield and mechanical robustness of the contact vias is also desirable. A need therefore exists for a means to reduce TFT photosensitivity without otherwise degrading imager performance, while also preserving or improving the integrity of the common electrode, the contact vias, and other imager structures.
The present invention includes a structure for a radiation imager comprising an opaque shield overlying substantially all of a photosensitive region of a switching device, which is disposed on a substrate. The photosensitive region comprises a light sensitive portion of a semiconductor layer, and this light sensitive portion overlies a bottom conductive metal layer, but is free of a first top conductive metal layer and a second top conductive metal layer, which overlie the semiconductor layer. Furthermore, the structure includes a light transmissive dielectric layer overlying the photosensitive region and a common electrode disposed between the light transmissive dielectric layer and the opaque shield.